Lodestones, not life

The building blocks for life emerged on Mars—but not life itself

FOR a dull lump of greyish rock, ALH 84001 has had an eventful life. The meteorite, which was retrieved from the Allan Hills of Antarctica in 1984, is certainly well travelled. Experts in the field think it came from Mars, having been blasted off the surface of that planet by a collision with an even bigger meteorite. More than that, it contains minerals that some researchers believed, in a flurry of publicity when the rock was properly examined just over a decade ago, must have been made by living things on the Martian surface. The number constituting “some” has been dwindling since then, but there are still a few hold-outs who think ALH 84001 is indeed the first evidence of extraterrestrial aliens—albeit of bacterial dimensions.

The reason why ALH 84001 is so interesting is that it contains organic compounds. Life is thought to have emerged on Earth from a primordial soup of such compounds, which are based on carbon, hydrogen and oxygen, though just how it did so is obscure. When ALH 84001 was analysed, however, some astrobiologists suggested that the conventional explanation might be wrong. Perhaps life had evolved on Mars first and the red planet had then “seeded” its blue neighbour when rocky projectiles similar to ALH 84001 were blasted off its surface in abundance in an era when the solar system had a lot of loose asteroids flying around.

To test these claims, a group of researchers led by Andrew Steele of the Carnegie Institution in Washington, DC, decided to identify precisely what ALH 84001 contains and then compare the results with those from rocks that definitely formed on Earth. Using a number of different techniques, they discovered that the tiny spheres of carbonate minerals within the meteorite which had caused such a stir were surrounded by an iron-oxide mineral called magnetite.

Some true believers had seen the presence of magnetite in the meteorite as evidence supporting their cause. That was because certain earthly bacteria make small grains of the mineral, in order to navigate their habitats using the Earth's magnetic field, much as human navigators once used lodestone compasses made of magnetite to find their way around. However, most of the magnetite on Earth is not bacterial. Instead, it was ejected in liquid form from volcanoes. If this magnetite cools in an environment rich in water and carbon dioxide (which terrestrial volcanoes usually are), it can act as a catalyst for the formation of organic compounds from the carbon, hydrogen and oxygen atoms in those two raw ingredients.

Dr Steele and his colleagues thus decided to compare ALH 84001 with volcanic rocks collected in Svalbard, the northernmost territory of Norway. These rocks are thought to have formed when volcanoes erupted in freezing, pristine conditions a million years or so ago.

The researchers found that the earthly boulders, too, contained carbonates encircled by magnetite. Writing in a forthcoming issue of Meteoritics and Planetary Science, they conclude that the organic material in ALH 84001 was made not by Martian microbes but, rather, by chemical reactions within the rock.

Such a conclusion may be disappointing to astrobiologists, who seek to study life on other planets, so far without success. But it also offers them hope. If Dr Steele and his colleagues are correct, then volcanic activity in a place other than Earth has produce interesting-looking organic compounds. That means the building blocks of life could form on cold, rocky volcanic planets throughout the universe. And the universe is a very big place.